MX2011009819A

MX2011009819A - Loudspeaker with passive low frequency directional control.

Info

Publication number: MX2011009819A
Application number: MX2011009819A
Authority: MX
Inventors: John D Meyer; Jean Pierre Marmin; Pablo Espinosa; Peter Kowalczyk
Original assignee: Meyer Sound Lab Inc
Priority date: 2009-03-20
Filing date: 2010-03-19
Publication date: 2012-01-20
Also published as: EP2409499A1; CN102422653B; WO2010108123A1; US8428284B2; US20100254558A1; CN102422653A; EP2409499A4; HK1169254A1

Abstract

A loudspeaker is provided that passively achieves a directional polar response at low frequencies with a high degree of attenuation between the front and the back of the loudspeaker. The loudspeaker 11 has a transducer 13, an enclosure 21 behind the transducer, and port openings 29 in the sidewalls 23 of the enclosure to allow a back- wave produced by the transducer to exit the enclosure and combine with the front wave produced by the transducer. Cancellation behind the loudspeaker at low frequencies is achieved by delaying the back wave with low loss. Low loss delay at low frequencies is achieved by inserting a low-density fibrous fill material 33 in the acoustic chamber 27 formed behind the transducer 13 by the enclosure 21. A fibrous material is selected having a low-pass transfer function and low acoustic loss in its low- frequency pass band.

Description

LOUDSPEAKER WITH LOW FREQUENCY PASSIVE DIRECTIONAL CONTROL DESCRIPTION OF THE INVENTION The present invention generally relates to loudspeakers, and more particularly relates to loudspeakers that claim to have a polar directional pattern through their operating frequency range, which includes low frequencies.

The term "polar pattern" refers to the distribution of acoustic energy through the space generated by the speaker, and is typically expressed in units of decibels (dB) as the magnitude of sound pressure in points on a circle or sphere around the speaker with respect to the sound pressure on the shaft (at zero degrees, directly in front of the transducer). At high frequencies where the wavelengths are small with respect to the diaphragm of the transducer, the speakers are naturally directional and often produce polar directional patterns described as "cardioid". (A true cardioid polar pattern can show a maximum sound pressure level on the axis in front of the speaker (zero degrees) and a pressure without any sound on the axis behind the speaker (180 degrees) Variations of a true cardioid pattern include "hypercardioid" and "supercardioid"). In addition, polar patterns at high frequencies can be easily manipulated with waveguides (horns). However, at low frequencies, where the wavelengths are larger than the diaphragm of the transducer, the speakers tend to generate omni-directional polar patterns. Extending the directional characteristics of a speaker down at low frequency margins presents a challenge for speaker designers.

A known method for producing directional characteristics in low frequency loudspeakers is to add secondary transducers which are optimized to cancel the acoustic energy created by the primary transducers of the loudspeaker in a desired region in space. Such cancellations result in a polar directional pattern. For example, it is known to employ two low frequency transducers, one of which operate normally, and the other in which it is optimized to cancel the acoustic energy produced by the first radiator in the region behind the loudspeaker, thus producing a cardioid or a near cardioid polar pattern at low frequency. This is achieved by actuating the second radiator with an inverted and delayed time polarity signal that has a different equalization than the first radiator, so that the desired points in the space of the radiator contributions are equal in magnitude and opposite in polarity. Similarly, polar directional patterns have been produced from the set of more than one radiator by providing a corresponding set of secondary radiators optimized to selectively cancel out the acoustic energy produced by the set of primary radiators. Although such "active" procedures have proven to be effective, two of the numbers of radiators and amplifiers are required, as well as complicated signal processing circuitry. They are therefore relatively expensive to manufacture.

Another method for achieving directional polar patterns over a wide frequency range is described in US Pat. No. 3,739,096. Here, a speaker system is described wherein a speaker enclosure is provided with cannulas covered by an acoustic insulation material that functions as an acoustic resistor. In this procedure, the slots, which effectively create a resonant conduit within the enclosure behind the diaphragm of the transducer, allow the acoustic pressure wave generated by the back of the diaphragm of the transducer to propagate out of the enclosure, where it can be combined with the acoustic energy in the frontal wave that is refracted around the speaker enclosure. To produce cancellations in such a passive procedure, the subsequent wave emerging from the speaker enclosure needs to be delayed due to differences in the front and back wave path length at the cancel point behind the speaker. It is well known that dampening an oscillation introduces a delay. However, damping also reduces the amplitude of the oscillation. Thus, while the cover slots or slots as described in US Pat. No. 3,739,096 may retard the back wave a bit, such a procedure is not very effective in producing high degrees of attenuation between the front and the back. of the speaker's polar pattern, achieving the best of a "sub-cardioid" response where the 180-degree cancellation is incomplete. To achieve high levels of attenuation, and thus a high degree of directionality, such as occurs in a true hypercardioid or cardioid response, the posterior wave emerges from an enclosure with ports not only must be retarded enough to reverse the polarity in the desired region of cancellation, it must have a magnitude that is substantially little attenuated with respect to the magnitude of the frontal wave that is canceled.

The present invention provides a loudspeaker that overcomes the disadvantages of the above methods for achieving directional control of the acoustic energy produced by the loudspeaker at low frequencies. The loudspeaker of the invention eliminates complexity and adds costs of active procedures to produce desired cancellations behind the loudspeaker, and provides a unique and effective procedure for producing highly directional polar patterns with high front and rear attenuations of the acoustic energy produced by the loudspeaker.

Briefly, the present invention provides a loudspeaker that passively achieves a polar directional response from the loudspeaker at low frequencies with a high degree of attenuation between the front and back of the loudspeaker. It is contemplated that attenuations in the order of minus 10 dB to 180 degrees can be achieved in a loudspeaker made in accordance with the invention having only one transducer. However, the invention is not limited to loudspeakers having a single transducer. It is contemplated that loudspeakers in accordance with the invention may include additional low frequency transducers, and that the invention may be incorporated into a loudspeaker system having high frequency transducers, for example, a high frequency conductor charged in the loudspeaker.

According to the invention, the high degree of forward-back attenuation is achieved by providing the loudspeaker with a louvred enclosure and filling at least a portion of the interior chamber of the enclosure with low density fibrous acoustic filler material placed behind the loudspeaker. loudspeaker transducer so that this extends substantially and preferably completely over the louvre openings of the enclosure. The acoustic filler material must be chosen to have particular acoustic properties, namely: it must have a low pass characteristic, and it must have a low loss characteristic in the desired low frequency range. The apertures with ports are preferably covered by a low loss acoustically transparent screen or screens. Such screens will contribute greatly to the suppression of the resonance of the enclosure in the resonant frequency of the enclosure. The grid screens can suitably be made of an aluminum sheet material having an adhesive backing (except the port areas) which allows the screens to join the surface of the side walls 23 of the enclosure.

By selecting a filler material for use in the enclosure, it is necessary to select and experiment with different fibrous materials to determine if they have the peculiar acoustic properties necessary to retard the posterior wave exiting the louvre openings of the speaker enclosure at low frequencies without attenuation. It has been discovered that a certain mineral wool exhibits the properties necessary for the filling material.

Various aspects of the invention will be apparent to those skilled in the art from the description of the illustrated mode.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1A is a graphical perspective view of a loudspeaker according to the invention, wherein a single transducer is mounted to an enclosure with ports.

Figure IB is another graphical perspective view of the same as shown by the loudspeaker with the screens over the port openings or the enclosure with lumens removed.

Figure 2 is a front elevational view thereof.

Figure 3A is a sectional view taken along lines 3-3 in Figure 2, showing the low density fibrous acoustic filler behind the transducer of the speaker provided in two layers.

Figure 3B is a sectional view taken along line 3-3 in Figure 2, showing the low density fibrous acoustic filler behind the transducer of the loudspeaker provided in a single layer.

Figure 4 is a circuit diagram showing a simplified and rough equivalent circuit of the speaker components shown in Figure 3.

Figure 5 is a perspective view of an exemplary embodiment of the speaker graphically illustrated in Figures 1-3.

Figure 6 is another perspective view thereof showing the loudspeaker with the screens over the openings with port or the enclosure with lumens removed.

Figure 7 is a side elevational view thereof with the side walls of the enclosure removed for illustrative purposes.

Figure 8 is an extreme elevation view of the speaker enclosure without the rear wall or the structure junctions that mount the transducer.

Figure 9 is a top perspective view of a block of the low density fibrous acoustic filling material used in the speaker enclosure.

Figure 10 is an exploded perspective view of another exemplary embodiment of a loudspeaker according to the invention having a different configuration for openings with ports in the loudspeaker enclosure.

Figure 11 is a schematic view of an exemplary test enclosure for evaluating the acoustic filler material through the enclosure with ports illustrated in Figures 3 and 7.

Figure 12 is a side elevational view of the screen block of low density fibrous acoustic filling material seen in Figure 9 showing a means for advantageously gripping the filling materials in the longitudinal direction.

Figure 13 is a front elevation view thereof.

With reference to the drawings, Figures 1A, IB, 2, 3A and 3B graphically illustrate a loudspeaker according to the invention, wherein the loudspeaker 11 is shown as including a single transducer 13 in the form of a cone radiator having a diaphragm 15 and the magnetic base assembly 17. The transducer, which produces acoustic energy from an electrical audio signal source, is mounted on the front end 19 of an enclosure 21 having side walls 23 and a rear wall 25. The cone driver can be mounted to the front end of the enclosure by mounting directly to the ends of the side walls of the enclosure, or can be mounted to a front baffle wall of the enclosure. With a relatively small speaker, the enclosure 21 can advantageously be manufactured from a single piece of machined extruded aluminum. In the illustrated embodiment, the enclosure has a square shape about its longitudinal axis. As a result, the side walls 23 of the enclosure have the same width.

The enclosure provides an internal acoustic chamber 27 behind the transducer 13, which contains a volume of air. The transducer 13 should be constructed with an open structure that exposes the back of its diaphragm 15 to this internal acoustic chamber. This will allow the posterior wave to be produced by the diaphragm of the transducer to propagate in the camera. It is noted that the components of the loudspeaker system may reside within the internal chamber 27, such as an electronic module 28 described in further detail in the following.

The enclosure 21 is substantially sealed, except for port openings 29. The port openings are located in the side walls 23 of the enclosure, and are preferably moved behind the transducer by a distance approximately equal to the diameter of the cone diaphragm of the transducer. The port openings are preferably provided on each side of the enclosure. Where more than one port opening is provided per side, as illustrated in Figures 1A and 6, where preferably it will be of an equal number of open ports per side. In any case, the port openings are preferably configured to provide a symmetrical distribution of the ports around the enclosure.

As seen in Figure 1, the ports on each side of the enclosure are preferably covered by screens 31 with grid that can be screens or grids. The main function of grid screens is to protect the inside of the enclosure from the entry of an unwanted object. Nevertheless, because it will also function as an acoustic resistor, it can also help dampen the resonance of the enclosure. Grid screens are preferably designed to introduce a minimum amount of loss when the acoustic energy passes through the screens. The total open area presented by the ports through the screen perforations, and consequently, the size of the port openings, is an important factor in achieving the cardioid polar response with a high degree of attenuation between the front and the part. rear of the speaker. As further discussed in the following, the size of the apertures in the ports to achieve an acceptable polar response is determined empirically.

The speaker system of the invention achieves significant attenuation of the acoustic energy directed behind the loudspeaker by using the back wave more effectively to cancel the frontal wave in a region in the space behind the loudspeaker. To achieve these desired cancellations, a time delay is necessary to compensate for the difference in travel time between the frontal wave, which diffracts around the enclosure, and the posterior wave, which leaves the port opening 29. Because the posterior wave travels a shorter, more direct path to the region behind the speaker, it must be delayed without significant attenuation in order to maintain the same magnitude and opposite polarity from the frontal wave and at the desired point of cancellation.

According to the invention, a low loss delay, required to achieve high forward-back attenuation in the polar pattern of the speaker at low frequencies, is achieved only by inserting a low density fibrous acoustic filler material, indicated by the number 33 in Figure 3A and 3B, in the acoustic chamber 27 of the loudspeaker behind the transducer 13. It will be discovered that an appropriately chosen low density fibrous filling material can effectively produce the required delay in the posterior wave without loss. To produce delay without loss at low frequencies, the filler material must have a low pass transfer function and low acoustic loss in its low frequency pass band. The acoustic filler material is preferably selected to exhibit the following characteristics: • The maximum low frequency attenuation through the material does not exceed -2dB, and • the material exhibits a transfer function approaching a first order of low pass filter with a corner frequency of approximately 700 Hz, with an attenuated magnitude and negative phase change (delay) with increasing frequency.

The acoustic filler material preferably fills a substantial portion of the interior acoustic chamber 27 of the enclosure 21, and preferably fills the entire chamber between the rear of the transducer 13 and a location that allows the filler material to cover at least substantially, and preferably, completely cover the port openings 29. Avoiding air spaces between the filler material and the side walls of the enclosure are considered important for the effectiveness of the filler material, when air spaces are likely to allow part of the acoustic energy to deviate from the filler material and compromise the resulting cancellations.

It has been found that providing a three inch thick block of Roxul AFB Mineral Wool in a four square inch enclosure behind a transducer having a diameter of 3.25 inches meets the above requirements. As shown in Figure 3A, three-inch thick mineral wool can be created by two back-to-back layers of 3.81 centimeters (1.5 inches) indicated by 33a and 33b. It must also be created by a higher number of thinner layers. However, it is found that a single layer produces the most consistent results in terms of the desired polar response. When using a single layer, the interconnections between the layers that can produce reflections at certain frequencies are eliminated.

It should be noted that not all fibrous materials of low density will have the characteristics necessary to achieve the objects of the invention. As discussed in more detail in the following, the determination of whether a particular low density fibrous material will meet the requirements of low pass and low loss can be determined empirically by an evaluation of the acoustic transfer function of the proposed material.

The low pass circuit equivalent of the speaker illustrated and described above is shown in Figure 4, where the transducer 13 is represented as a source of signal S, the enclosure 21 is represented by a parallel capacitor C, the low density fibrous acoustic filling material 33 having the characteristics described in the foregoing is represented as the resistors Ri in series, the ports 29 of the enclosure are represented as an inductor L in series, and the screen 31 on the port openings is represented as a resistor R2 in series. Similar to an electric low pass filter as shown in Figure 4, which delays the signal from the signal source S as this passes to the filter output terminals T, the physical low pass filter provided by the indicated components in the foregoing of the loudspeaker 11 delays the low frequency acoustic pressure waves passing through the fibrous filling material and the port openings without attenuating the subsequent wave. With the necessary delay performed by a suitable low density fibrous filling material, the delay introduced by the screens 31, which cover the port openings 29, should be minimized to avoid excessive delay. The screens are preferable and acoustically transparent, without acoustic loss that is introduced by the screens.

It is further noted that the low pass filter characteristics of the acoustic filler material 33 means that the high frequency energy is filtered from the subsequent wave. However, because the loudspeaker becomes directional at high frequencies, the cancellation of acoustic energy behind the loudspeaker at high frequencies is unnecessary. In fact, this characteristic of the filling material provides several advantages. First, the substantial removal of the high frequency energy in the posterior wave will substantially eliminate the detrimental effect that such energy could have on the total polar patterns. Second, the acoustic filler material also serves to substantially dampen the resonance of the enclosure. Without this damping effect, the posterior wave could excite the resonance of the enclosure, which results in an excess acoustic energy at that resonant frequency. This excess energy could overwhelm the frontal wave and prevent subsequent cancellation of the resonant frequency. Finally, the acoustic filler material can be used to reduce the cross-sectional area of the closure to approximately the same area as the diaphragm of the transducer. By keeping the area approximately the same as that of the diaphragm of the transducer, it has been found that the attenuation of the recoil wave as it leaves the enclosure can be maintained at less than one dB.

Figures 5-9 show a physical implementation of the loudspeaker system depicted graphically in Figures 1, 2, 3A and 3B, wherein the transducer 13 has a square mounting structure 35 fastened to the front end 19 of the enclosure 21 by means of screws 37 of the corner, whose screws in the screw channels 42 in the corner projections 40 formed in the interior of the corners of the side walls 23 of the enclosure are shown in Figure 8. Figure 9 shows a cut-off block for the die. an acoustically suitable low density fibrous filling material 33, such as a 3 inch thick block of Roxul AFB Mineral Wool. It is noted that the perimeter block 33 conforms to the internal cross-sectional shape of the loudspeaker enclosure 21, which includes the provision of corner cutting notches 34 which fits over the internal corner projections 40 of the enclosure. The filler block is inserted into the enclosure 21 behind the transducer and the mounting structure of the transducer and will preferably fill the enclosure from the rear of the transducer magnetic assembly 17 at a location behind the port opening 29.

As best seen in Figure 7, the rear wall 25 of the enclosure supports the internal electronic module 28, as well as the external heat radiation fins 39 and the electrical connectors 41. The port openings 29 are preferably distributed around the side walls 23 of the enclosure at a distance behind the diaphragm of the radiator approximately equal to the diameter of the transducer. The Screens 31 are seen to be inserted into the elongated recess areas 30 in the walls surrounding the port openings. Each of the grid screens can be suitably manufactured from aluminum foil in the form of screen plates having a configuration conforming to the recessed areas. An adhesive backing may be provided on the back of the screen plates (except in the area of the port openings) to allow the screens to be secured in the recesses of the enclosure.

The total opening area of the louvre openings 29 as restricted by the perforated screens 31 has a substantial effect on the loudspeaker's polar response. In this way, the dimension of the lumbreras is important to achieve acceptable cardioid functioning. A port opening design that achieves an acceptable cardioid polar response can be determined empirically by trial and error. For example, in the case of a loudspeaker having three circular lumen openings, as shown in Figures IB and 6, one enclosure 21 of ten point sixteen centimeters by ten point sixteen centimeters (four inches by four inches) wide, the enclosure filled with two layers of 3.81 cm (1 1/2 inch) thickness of Roxul AFB Mineral Wool, and a transducer having a transducer diaphragm diameter of 8.255 m (3.25 inches), it has been found that the following attributes of the Louver and screen openings produce an acceptable cardioid polar response at low frequencies: • the total area of the luminaries per side is 8.54837 cm2 (1.325 square inches), for a total of 34.19348 cm2 (5.3 square inches) for all four sides; • the screen plates are made of aluminum foil material having a thickness of 0.127 cm (0.05 inches) and an open area drilling of approximately 19%, resulting in a total open area through the perforations for all four sides of the speaker enclosure of approximately 6,496,761 cm2 (1,007 square inches).

As further indicated in the following, it will be understood that different screen materials and port opening sizes and configurations can be used, that will not be unacceptably degraded the desired polar patterns and the low frequency response of the loudspeaker.

As noted above, the invention contemplates the possibility of using more than one transducer. When several transducers of different sizes are used, it is contemplated that the larger transducer will dictate the space between the transducers and the ports of the enclosure. In such a case, it is also contemplated that the total combined open area of the ports will be dictated by the surface area of the radiant portion of the larger transducer. As in the illustrated modes, the size of the ports required to achieve an acceptable polar response can be determined empirically.

The speaker enclosure 21 shown in Figures 5-8 is also seen to include suitable mounted hardware, such as the illustrated post connector 43 (which projects into the enclosure as shown in Figure 8), and an acoustically grate 36 transparent to cover the diaphragm of the transducer 13. The block of fibrous material 33 can be suitably extended from the back of the transducer to the pole connectors projecting internally.

The electronic module 28 at the rear of the internal chamber 27 of the enclosure is suitably an integrated electronic package containing an amplifier and circuitry that processes the signal. The electronic package of preference accepts a balanced audio signal through the connector 41 from an audio mixer or other source, which can be modified and processed as follows: • Frequencies outside the intended operating range of the loudspeaker (for example, frequencies or a CD offset) are filtered.

• The frequency response of the compound is configured to compensate for variations in the response of the actuator and the enclosure, which result in a generally flat response within the intended operating range.

"The peak amplitude of the signal is limited with a fast time constant to minimize the clipping of the amplifier, which results in increased harmonic distortion.

• The RMS voltage of the signal is limited to a slow time constant to protect the transducer from damage due to overheating.

• The signal is amplified by a power amplifier stage capable of driving the relatively low impedance transducer.

When the invention is represented as a miniature loudspeaker system where efficiency is essential to produce adequate sound pressure levels, the amplifier stage can be advantageously implemented using Class D amplifier technology (pulse width modulation).

Figure 10 shows an alternative to the embodiment of the speaker system described above, wherein, instead of having multiple openings, each of the side walls 23 of the enclosure 21 has a single elongated port opening 36 preferably having rounded ends 38 to form a port in the form of an elongated race track. (Rounded ends prevent undesirable refraction that can occur around the square corner surface). This open port configuration increases the total area of the port openings, when compared to the previously described modes and has been found to improve the low frequency cardioid polar pattern of the loudspeaker.

For example, in the case of a loudspeaker having a runner opening in the form of a running track as shown in Figure 10, an enclosure 21 of ten point sixteen centimeters by ten point sixteen centimeters (four inches by four inches) of width filled with a 7.62 cm (3 inch) block of Roxul AFB Mineral Wool, and a transducer that has a diameter of the transducer diaphragm of 8.255 cm (3.25 inches), it has been found that the following attributes of lumen openings and the screen plates produce an improved cardioid polar response at low frequency when compared to the configuration of three ports per side port shown in Figures 1 and 6: • the total area of each port per side is 15.7419 cm2 (2.44 square inches) for a total of 62.86762 cm2 (9.76 square inches) for all four sides of the enclosure; • the screen plates 21 are made of aluminum foil material having a thickness of 0.127 cm (0.05 inches) and an open area of perforation of approximately 19%, resulting in a total open area through the perforation for the four sides of the speaker enclosure approximately 11.93546 cm2 (1.85 square inches).

Similar to the embodiments described in the foregoing, the raceway shaped openings 36 shown in Figure 10 are distributed around the side walls 23 of the speaker enclosure preferably at a distance behind the diaphragm of the transducer which approximately equals to the diameter of the diaphragm. The screens 31 (not shown in Figure 10) are inserted into the elongated recess areas 30 in the side walls to cover the port openings, and serve the functions described above.

Filling Material Evaluation In order to determine whether a particular fibrous material meets the requirements for the filling material of the loudspeaker 33 (which exhibits a low pass filtering characteristic with low losses in the bandpass) the acoustic transfer function of the candidate material must be determined. With reference to Figure 11, the measurements required to determine a material transfer function can be made using a test enclosure 45 having an exterior enclosure 47, an interior sealed enclosure 49, and a transducer 51 mounted on the front of the enclosure inside. The inner sealed enclosure and the radiator can suitably be an existing loudspeaker maintained within the outer enclosure. To provide acoustic insulation around the speaker, the outer enclosure can be filled with a suitable insulating foam material 52.

The test enclosure is seen to have a front opening 53 through which the sound waves produced by the transducer 51 can propagate. This opening is provided by a structure 55 that holds the sample material in the front of the outer enclosure, which physically holds the sample material to be measured (indicated by number 57) on the front of the transducer 51. The opening in the The structure that holds the sample is formed to firmly hold the material samples without gaps between the clamping structure and material samples. For example, the fastening structure can suitably be a square ring having a square opening to hold a square piece of material. The opening is preferably similar in size or slightly larger than the diaphragm of the transducer and, preferably, has a depth that allows material samples to be fully adapted within the fastening ring. When surrounding the complete sample, the clamping ring will prevent the acoustic energy produced by the transducer from propagating away from the sides of the sample, and thus compromising the measurement.

To evaluate a sample of material using test enclosure 45, the transfer function of the test enclosure without a sample of material must first be measured. The material sample 57 is then placed in the openings 53 of the structure 55 that holds the sample of the test enclosure, and the combined transfer function of the test enclosure with the measured sample. Measurements of the transfer function can be made using a microphone 59 placed on the axis in a meter in front of the test room. The microphone 59 is connected to a sound analyzer, such as the commercially available SIM 3 sound analyzer manufactured by eyer Sound Laboratories, Incorporated, which determines the transfer functions.

Using the measured transfer functions, the transfer function of the material sample can be determined. If the transfer function of the test enclosure without the sample material is indicated H (s), and the transfer function of the test enclosure with the sample material is indicated G (s), then G (s) = H (s) * M (s) (1) where M (s) is the transfer function of the material sample. To determine M (s), the second measure is normalized in the first measure: M (s) = G (s) * H (s) (2) When G (s) and H (s) are complex frequency response vectors, this process is performed numerically by dividing the two factors as shown in equation (2). The transfer functions M (s) n of a collection of candidate materials can now be analyzed to identify which candidates are likely to perform well on a passive cardioid speaker.

Impingement of Filler Material As previously described, the selected fibrous filler material can be cut into a material block 33 that fits within the speaker enclosure 21 without substantial spaces between the filler material and the side walls 23 of the enclosure. It has been found that forward-back attenuation in the polar response can improve little by compressing the filled material in the longitudinal direction without obstructing the acoustic path through the material.

The means for compressing the filler material in the longitudinal direction are shown in Figures 12 and 13. In these figures, the block of filler material 33 having corner notches 34 is imprisoned in the longitudinal direction by opposite trapping screens 61. - suitably metal screens - which are held against opposite surfaces 62 of the block of filler material by a bolt 63. The bolt preferably extends through the center of the material and can be adequately secured by the nut 65, which can tighten to achieve an adequate degree of compression of the filling material. The degree of compression needed to achieve optimal improvements in forward-back attenuation in the speaker's polar response can be determined empirically.

The screens 61 preferably cover the majority of the surfaces 62 the block 33 of fibrous material to provide uniform compression through the block of material. They also preferably have minimal resistance to the acoustic pressure waves that must pass through it, a feature that can be obtained by securing a large percentage of the open area. It is believed that screens having an open area percentage of at least about 40% to 50% may be required to achieve adequate results, however, lower percentages may be possible. Suitable screens can be selected empirically by testing different commercially available screen materials.

Although the present invention has been described in considerable detail in the above specification and the accompanying drawings, it will be understood that the invention is not intended to be limited to such details, unless expressly indicated. Other embodiments of the invention not expressly described herein may be readily apparent to those skilled in the art of this disclosure.

Claims

1. A loudspeaker with passive directional control at low frequencies, characterized in that it comprises: an enclosure that has a front, a back and side walls, a transducer mounted on the front of the enclosure, the transducer that has a diaphragm to produce acoustic energy from an electrical audio signal, the acoustic energy being produced as frontal waves and subsequent waves, an acoustic chamber in the room behind the transducer to receive subsequent waves produced by the transducer, at least one port opening in such an enclosure through which the acoustic energy in the acoustic chamber produced by the transducer can exit the enclosure, and an acoustic filler material in the acoustic chamber that substantially fills the entire internal acoustic chamber of the enclosure between the side walls thereof and which extends from around the transducer for a distance that substantially covers at least one port opening, the acoustic filler material having a low pass, low loss transfer function, wherein the subsequent waves introduced into the acoustic chamber by the experience delay of the transducer at low frequencies with minimal attenuation before it leaves the enclosure through at least one port opening.

2. The speaker in accordance with the claim 1, characterized in that the port openings are provided in at least one side wall of the enclosure.

3. The loudspeaker according to claim 1, characterized in that at least one port opening is provided in each side wall of the enclosure.

4. The loudspeaker according to claim 1, characterized in that a single port opening is provided in each side wall of the enclosure.

5. The loudspeaker according to claim 4, characterized in that the port opening has an elongated raceway shape.

6. The loudspeaker according to claim 1, characterized in that the loudspeaker has a polar response characteristic, and wherein the total area of the louvre opening is selected empirically to achieve a high degree of attenuation in the polar response at the rear of the loudspeaker. speaker.

7. The loudspeaker according to claim 1, characterized in that the acoustic filler material is a low density fibrous material having a low pass, low loss transfer function.

8. The loudspeaker according to claim 7, characterized in that the low density fibrous material is a mineral wool having a low pass, low loss transfer function.

9. The loudspeaker according to claim 1, characterized in that the acoustic filler material extends from the transducer to cover at least substantially at least one port opening in the enclosure.

10. The loudspeaker according to claim 1, characterized in that the acoustic filler material has a maximum low frequency attenuation through the material no greater than about -2dB.

11. The speaker in accordance with the claim 1, characterized in that the transfer function of the filler material approaches a first order low pass filter having a corner frequency of about 700 Hz.

12. The speaker in accordance with the claim 1, characterized in that the transfer function of the filling material exhibits attenuated magnitude and negative phase change with increased frequency.

13. The loudspeaker according to claim 1, characterized in that at least one port opening is covered by a substantial acoustically transparent screen element.

14. The loudspeaker according to claim 1, characterized in that the acoustic filler material is clamped in the longitudinal direction in the enclosure to compress the acoustic filler material in the longitudinal direction.

15. The loudspeaker according to claim 1, characterized in that the acoustic filler material is provided in a single block of acoustic filler material.

16. The loudspeaker according to claim 1, characterized in that at least one port opening is covered by a grid screen that provides an acoustic resistance in the port opening.

17. A loudspeaker with passive directional control at low frequencies, characterized in that it comprises: an enclosure that has a front, a back and side walls, a transducer mounted on the front of the enclosure, the transducer that has a diaphragm to produce acoustic energy from an electrical audio signal, the acoustic energy being produced as frontal waves and subsequent waves, an acoustic chamber in the room behind the transducer to receive subsequent waves produced by the transducer, at least one port opening in each of the side walls of the enclosure through which acoustic energy in the acoustic chamber produced by the transducer can exit the enclosure, a grid screen covering each of the port openings , grid screens that provide low loss acoustic resistance at each port opening, and an acoustic filler material in the acoustic chamber, the acoustic filler material having a low pass transfer function, low loss, wherein the subsequent waves introduced into the acoustic chamber by the experience delay of the transducer with minimal attenuation before that leaves the enclosure through at least one port opening,. the acoustic filler material substantially fills the entire internal acoustic chamber of the enclosure between the side walls thereof and which extends from around the transducer for a distance that substantially covers a port opening.

18. The loudspeaker according to claim 17, characterized in that the port openings are positioned behind the transducer for a distance approximately equal to the diaphragm diameter of the transducer cone.

19. The speaker in accordance with the claim 18, characterized in that the port openings are configured to provide a symmetrical distribution of ports around the enclosure.

20. The speaker in accordance with the claim 19, characterized in that the single elongated port openings are provided in each side wall of the enclosure.

21. The speaker in accordance with the claim 20, characterized in that the single port has rounded ends.

22. A loudspeaker with passive directional control at low frequencies, characterized in that it comprises: an enclosure that has a front, a back and side walls, a transducer mounted on the front of the enclosure, the transducer that has a diaphragm to produce acoustic energy from an electrical audio signal, the acoustic energy being produced as frontal waves and subsequent waves, an acoustic chamber in the room behind the transducer to receive subsequent waves produced by the transducer, at least one port opening in each of the side walls of the enclosure through which the acoustic energy in the acoustic chamber produced by the transducer can exit the enclosure, a means for providing a low loss acoustic resistance in each opening of louvre, and an acoustic filler material in the acoustic chamber that substantially fills the entire interior acoustic chamber of the enclosure between the side walls thereof and which extends from around the transducer to a distance that substantially covers at least one opening loudspeaker, and having a maximum low frequency attenuation through the material no greater than about -2dB, and a transfer function approaching a first order low pass filter having a corner frequency of about 700Hz.

23. The loudspeaker according to claim 22, characterized in that the acoustic filler material is clamped in the longitudinal direction in the enclosure substantially from one side wall of the enclosure to another to substantially compress all of the acoustic filler material in the longitudinal direction.

24. The loudspeaker according to claim 22, characterized in that the acoustic filler material is provided in a single block of acoustic filler material.